The instant invention is directed to a battery separator having improved pin removal properties.
In the manufacture of high energy, lightweight batteries, for example, secondary lithium batteries, the battery assembly, i.e., an anode tape and a cathode tape sandwiching a separator tape, is wound about one (1) or more pins (or cores or mandrels) . To begin winding of the assembly, the separator tape is taken up on the pin, and then the anode and cathode tapes are fed to the pin. Upon completion of the winding, the battery assembly is removed (or withdrawn) from the pin. If the assembly (i.e., the separator tape) sticks on the pin during withdrawal, the assembly xe2x80x9ctelescopesxe2x80x9d and must be rejected. Such rejects increase the cost of the battery manufacturing process. Accordingly, battery manufacturers desire separator tapes that have improved pin removal properties, i.e., separators that will not stick to the pin when the battery assembly is removed therefrom.
Several attempts have been made to solve the foregoing pin removal problem. In Japanese Kokai 10-110052 published Apr. 28, 1998, the surface of a microporous membrane is textured to improve the pin removal. Spherical particles, consisting of an organic resin, preferably at least one resin selected from fluororesins and silicone resins, protrude from the membranes surface. The textured surface reduces frictional resistance. In Japanese Kokai 10-139918 published May 26, 1998, the surface of a microporous membrane is coated with a lubricant to improve pin removal. The coating may be accomplished by dip coating or roll coating. The lubricants include (waxes e.g., paraffin wax, microcrystalline wax, low-molecular weight polyethylene and other hydrocarbons waxes); fatty acid esters (e.g., methyl stearate, stearyl stearate, monoglyceride stearate); aliphatic amides (e.g., stearamide, palmitamide, methylene bis stearamide), and combinations thereof. In Japanese Kokai 10-195215 published Jul. 28, 1998, the surface portion of the microporous membrane, which has a greater ratio of polyethylene to polypropylene than the middle portion of the membrane, has improved pin removal properties. In U.S. patent application Ser. No. 09/661,519 filed Sep. 13, 2000, the surface of the pin is modified to improve pin removal. The surface modifications include texturing (roughing) the surface and/or grooving the surface of the pin.
The use of calcium stearate as an additive in polyolefin resins is known. See: Plastics Engineering Handbook, Chapman and Hall, New York City, N.Y., (1991), p. 645. Therein, it is noted that calcium stearate, when used in polyolefins, acts to xe2x80x98tie up catalyst,xe2x80x99 that lubricants are used to enhance resin processibility, and that xe2x80x98effective lubricants . . . do not adversely affect the properties of end productsxe2x80x99 (i.e., they are inert). For example, 600 ppm of calcium stearate has been added to polypropylene for the purpose of acting as an acid scavenger. See Witco""s Additive Product Guide, Polymer Additives Group, Olefins/Styrenics, page 2. Calcium stearate may also act as a lubricant to improve the flow characteristics of the polyolefin resins. Witco, Ibid., page 2. Polypropylene resins containing calcium stearate have been used to make battery separators, i.e., CELGARD(copyright) 2400, a single layer polypropylene separator, and CELGARD(copyright) 2300, a multilayered separator.
In spite of the foregoing efforts, there is still a need to improve the pin removal properties of these separators.
A method for removing a pin from a battery assembly comprising the step of providing a separator comprising:
a microporous membrane having an exterior surface portion of polypropylene, the polypropylene including at least 50 ppm of metallic stearate and being adapted to exhibit a pin removal force xe2x89xa67100 g.
A microporous membrane, as used herein, has a plurality of micropores that extend through the membrane. The micropores have an average pore size ranging from 0.005 to 10 microns, preferably 0.01 to 5 microns, and most preferably 0.05 to 2 microns. The microporous membrane also has a Gurley (ASTM D726B) ranging from 5 to 100 seconds, preferably 10-60 seconds. Preferably, these membranes are xe2x80x9cshutdown membranes,xe2x80x9d i.e., having the ability to stop ion flow between the anode and cathode upon the onset of a rapid increase in the battery""s temperature due to, for example, internal short circuiting. The microporous membrane may be a single ply membrane or a multilayered membrane, as is well understood in the art. The preferred multilayered structure has a polypropylene/polyethylene/polypropylene structure (PP/PE/PP). The preferred single ply membrane is made from polypropylene. The exterior surface portion of the membrane (a coined term that refers to the surface of either a single ply or multi ply membrane) is preferably a polypropylene. The polypropylene is preferably an isotactic polypropylene homopolymer. Such polypropylene has a melt flow index (MFI) ranging from 1 to 4, preferably, 1.2 to 1.7. Such polypropylene has a density ranging from 0.90 to 0.91 g/cm3. Preferably, these membrane""s are produced by an xe2x80x98extrude, anneal, stretchxe2x80x99 method (a/k/a the xe2x80x98dry stretchxe2x80x99 or the Celgard(copyright) method), but may include other techniques such as xe2x80x98solvent extraction.xe2x80x99 See: Kesting, R., Synthetic Polymeric Membranes, John Wiley and Sons, New York City, N.Y. (1985), incorporated herein by reference.
It has been determined that the pin removal properties of the separator are improved by the inclusion of a metallic stearate in the polypropylene exterior surface portion of the membrane, preferably at least 50 ppm of the metallic stearate. The preferred metallic stearate is calcium stearate. Preferably, the calcium stearate in the polypropylene is at least 50 ppm, preferably 50-3000 ppm, and most preferred, between 300 and 600 ppm. The amount of calcium stearate was determined by induced coupled plasma (ICP) analysis. The lower limit defines the minimum amount of metallic stearate needed to obtain the improved pin removal properties. The upper limit defines the maximum amount of the metallic stearate tolerable before the risk, that the metallic stearate will dissolve from the separator in to the electrolyte and thereby adversely impact the electrochemical properties of the electrolyte, becomes to great. Such calcium stearates are commercially available from Witco Corporation, Greenwich, Conn. under the tradename of xe2x80x98Calcium Stearate Regular.xe2x80x99
The present invention will be further explained with reference to the examples set forth below.
The pin removal properties were quantified using the following procedure that measures the xe2x80x98pin removal force (g).xe2x80x99
A battery winding machine was used to wind the separator around a pin (or core or mandrel). The pin is a two (2) piece cylindrical mandrel with a 0.16 inch diameter and a smooth exterior surface. Each piece has a semicircular cross section. The separator, discussed below, is taken up on the pin. The initial force (tangential) on the separator is 0.5 kgf and thereafter the separator is wound at a rate of ten (10) inches in twenty four (24) seconds. During winding, a tension roller engages the separator being wound on the mandrel. The tension roller comprises a ⅝xe2x80x3 diameter roller located on the side opposite the separator feed, a xc2xexe2x80x3 pneumatic cylinder to which 1 bar of air pressure is applied (when engaged), and a xc2xcxe2x80x3 rod interconnecting the roller and the cylinder.
The separator consists of two (2) 30 mm (width)xc3x9710xe2x80x3 pieces of the membrane being tested. Five (5) of these separators are tested, the results averaged, and the averaged value is reported. Each piece is spliced onto a separator feed roll on the winding machine with a 1xe2x80x3 overlap. From the free end of the separator, i.e., distal the spliced end, ink marks are made at xc2xdxe2x80x3 and 7xe2x80x3. The xc2xdxe2x80x3 mark is aligned with the far side of the pin (i.e., the side adjacent the tension roller), the separator is engaged between the pieces of the pin, and winding is begun with the tension roller engaged. When the 7xe2x80x3 mark is about xc2xdxe2x80x3 from the jellyroll (separator wound on the pin), the separator is cut at that mark, and the free end of the separator is secured to the jellyroll with a piece of adhesive tape (1xe2x80x3 wide, xc2xdxe2x80x3 overlap). The jellyroll (i.e., pin with separator wound thereon) is removed from the winding machine. An acceptable jellyroll has no wrinkles and no telescoping.
The jellyroll is placed in a tensile strength tester (i.e., Chatillon Model TCD 500-MS from Chatillon Inc., Greensboro, N.C.) with a load cell (50 lbsxc3x970.02 lb; Chatillon DFGS 50). The strain rate is 2.5 inches per minute and data from the load cell is recorded at a rate of 100 points per second. The peak force is reported as the pin removal force.
Gurley was measured according to ASTM-D726(B). COF (Coefficient of friction) Static was measured according to JIS P 8147 entitled xe2x80x9cMethod for Determining Coefficient of Friction of Paper and Board.xe2x80x9d